This invention relates generally to magnetic resonance data acquisition and imaging, and more particularly the invention relates to a three-dimensional magnetic resonance sequence for acquiring volumetric data depicting a motion cycle.
Cardiac MRI can provide extremely accurate measures of chamber volumes, stroke volume, cardiac output and ejection fraction. The most commonly used methods are cine MR (described in U.S. Pat. No. 4,710,717, "Method for Fast Scan Cine NMR Imaging," Norbert J. Pelc and Gary H. Glover, December, 1987), and segmented k-space techniques (described in U.S. Pat. No. 5,377,680, "MRI Cardiac Image Produced by Temporal Data Sharing," Tsur Bernstein and Thomas K. Foo, January, 1995). Both of these techniques employ two-dimensional (2D) data acquisition strategies, and have limitations. For example, a cine scan with 128 phase-encodings can image 3 slices with a temporal resolution of approximately 60 ms in roughly 2 minutes, but the need for volumetric coverage increases the total scan time substantially. The segmented k-space methods reduce the scan time at the expense of temporal resolution. They image a single slice in a breath-hold with 80 ms temporal resolution. However, the rest periods between scans makes the total scan time for volumetric coverage comparable to that in cine. Both techniques produce 2D image sets, which must be concatenated for volumetric measurements. Thus, any subject movement during the scan can lead to slice registration errors, which in turn can affect the quality of the results. With both techniques, it may be difficult to obtain high spatial resolution in the slice direction. Further, both methods can suffer from insufficient contrast between the blood in the chambers and the myocardium, especially during flow stasis in diastole. This makes automatic volumetric analysis difficult, and manual analysis is tedious and introduces measurement variability.
MRI techniques produce images of individual slices by encoding spatial information. In the most commonly used method, often called 2DFT or "spin-warp", location in one direction is encoded using selective excitation which generates signal only from a slice through the object. Location in a second direction is encoded by acquiring the signal in the presence of a magnetic field in that direction, thereby encoding position into the temporal frequency of the measured signal. Location in the third direction is encoded using a preparation "phase encoding gradient". To form an image, the sequence of pulses that form the pulse sequence must be repeated many times using many values of this phase encoding gradient. The time between sequence repetitions is called the repetition time TR.
The method described above forms an image of a single plane. Many such images to cover a volume can be acquired either sequentially or in an interleaved manner. It is also possible to acquire data from a 3D volume simultaneously. For example, one can generate signal using an excitation pulse that excites a relatively thick slab. Position in the slice direction along the slab is encoded using a second phase encoding gradient.
Imaging techniques which are faster than those described above are known. One, called Echo Planar Imaging (EPI) is even capable of forming an image of an entire slice from data acquired during a single pulse sequence execution. However, the image quality using "single shot EPI" may not be acceptable for some applications. For these, it is possible to use multi-shot EPI. Other fast imaging methods, e.g. spiral k-space scanning, are also known.